[ZODB4] Backport the way MVCC is handled from ZODB5

This backports to ZODB4 Connection ZODB5's approach to handle MVCC via
always calling storage.loadBefore() instead of "load for latest version
+ loadBefore if we were notified of database changes" approach.

Why?
----

Short answer: because Wendelin.core 2 needs to know at which particular
database state application-level ZODB connection is viewing the
database, and it is hard to implement such functionality correctly
without this backport. Please see appendix for the explanation.

What
----

This backports to ZODB4 the minimum necessary part of upstream commit 227953b9
(Simplify MVCC by determining transaction start time using lastTransaction) +
follow-up correctness fixes:

https://github.com/zopefoundation/ZODB/issues/50
https://github.com/zopefoundation/ZODB/pull/56
https://github.com/zopefoundation/ZODB/pull/291
https://github.com/zopefoundation/ZODB/pull/307

In short:

- a Connection is always opened with explicitly corresponding to a particular database revision
- Connection uses only loadBefore with that revision to load objects
- every time a Connection is (re)opened, the result of queued invalidations and
  explicit query to storage.lastTransaction is carefully merged to refresh
  Connection's idea about which database state it corresponds to.

The "careful" in last point is important. Historically ZODB5 was first reworked
in commit 227953b9 (https://github.com/zopefoundation/ZODB/pull/56) to always
call lastTransaction to refresh state of Connection view. Since there
was no proper synchronisation with respect to process of handling
invalidations, that lead to data corruption issue due to race in
Connection.open() vs invalidations:

https://github.com/zopefoundation/ZODB/issues/290

That race and data corruption was fixed in commit b5895a5c
(https://github.com/zopefoundation/ZODB/pull/291) by way of avoiding
lastTransaction call and relying only on invalidations channel when
refreshing Connection view.

This fix in turn led to another data corruption issue because in
presence of client-server reconnections, ZODB storage drivers can partly
skip notifying client with detailed invalidation messages:

https://github.com/zopefoundation/ZODB/pull/291#issuecomment-581047061

A fix to that issue (https://github.com/zopefoundation/ZODB/pull/307)
proposed to change back to query storage for lastTransaction on every
Connection refresh, but to implement careful merging of lastTransaction
result and data from invalidation channel. However it was found that the
"careful merging" can work correctly only if we require from storage
drivers a particular ordering of invalidation events wrt lastTransaction
return and result:

https://github.com/zopefoundation/ZODB/pull/307#discussion_r434145034

While ZEO was already complying with that requirements, NEO had to be
fixed to support that:

https://github.com/zopefoundation/ZODB/pull/307#discussion_r434166238
neoppod@a7d101ec
neoppod@96a5c01f

Patch details
-------------

We change Connection._txn_time to be a "before" for the database state
to which Connection view corresponds. This state is hooked to be
initialized and updated in Connection._flush_invalidations - the
function that is called from both explicit Connection (re)open and at
transaction boundaries via Connection.afterCompletion hook.

Objects loading is factored into Connection._load which replaces old
"load + check invalidated + fallback to loadBefore" game in
Connection._setstate.

Connection.open now calls Connection._flush_invalidations
unconditionally - even if it was global cache reset event - because
besides invalidation flushes the latter is now responsible for querying
storage lastTransaction.

TmpStore - a "storage" that provides runtime support for savepoints - is
refactored correspondingly to delegate loading of original objects back
to underlying Connection.

DB.close is modified - similarly to ZODB5 - to release DB's Connections
carefully with preventing connections from DB poll from implicitly
starting new transactions via afterCompletion hook.

ZODB.nxd_patches is introduced to indicate to client software that this
particular patch is present and can be relied upon.

Tests are updated correspondingly. In 227953b9 Jim talks about
converting many tests - because

	"Lots of tests didn't clean up databases and connections properly"

and because new MVCC approach

	"makes database and connection hygiene a bit more important,
	especially for tests, because a connection will continue to interact
	with storages if it isn't properly closed, which can lead to errors if
	the storage is closed."

but finally implementing automatic cleanup at transaction boundaries
because there are too many tests to fix. We backport only automatic
cleanup + necessary explicit test fixes to keep the diff minimal.

All tests pass. This includes tests for ZODB itself, ZEO and NEO test
over hereby modified ZODB(*), my test programs from

https://github.com/zopefoundation/ZODB/issues/290	and
https://github.com/zopefoundation/ZEO/issues/155

and ERP5 tests. Upcoming wendelin.core 2 also work with this change.

(*) ZEO, NEO and ERP5 tests fail sometimes, but there is no regression
here because ZEO, NEO and ERP5 tests are failing regularly, and in the
same way, even with unmodified ZODB.

Appendix. zconn_at
------------------

This appendix provides motivation for the backport:

For wendelin.core v2 we need a way to know at which particular database
state application-level ZODB connection is viewing the database. Knowing
that state, WCFS client library interacts with WCFS filesystem server
and, in simple terms, requests the server to provide data as of that
particular database state. Let us call the function that for a client
ZODB connection returns database state corresponding to its database
view zconn_at.

Now here is the problem: zconn_at is relatively easy to implement for
ZODB5 - see e.g. here:

https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-54-ga6a8f5b/lib/zodb.py#L142-181
wendelin.core@3bd82127

however, for ZODB4, since its operational models is not
directly MVCC, it is not that straightforward. Still, even for older
ZODB4, for every client connection, there _is_ such at that corresponds
to that connection view of the database.

We need ZODB4 support, because ZODB4 is currently the version that
Nexedi uses, and my understanding is that it will stay like this for not
a small time. I have the feeling that ZODB5 was reworked in better
direction, but without caring enough about quality which resulted in
concurrency bugs with data corruption effects like

https://github.com/zopefoundation/ZODB/issues/290
https://github.com/zopefoundation/ZEO/issues/155
etc.

Even though the first one is now fixed (but it broke other parts and so
both ZODB had to be fixed again _and_ NEO had to be fixed for that ZODB
fix to work currently), I feel that upgrading into ZODB5 for Nexedi will
require some non-negligible amount of QA work, and thus it is better if
we move step-by-step - even if we eventually upgrade to ZODB5 - it is
better we first migrate wendelin.core 1 -> wendelin.core 2 with keeping
current version of ZODB.

Now please note what would happen if zconn_at gives, even a bit, wrong
answer: wcfs client will ask wcfs server to provide array data as of
different database state compared to current on-client ZODB connection.
This will result in that data accessed via ZBigArray will _not_
correspond to all other data accessed via regular ZODB mechanism.
It is, in other words, a data corruptions.
In some scenarios it can be harmless, but generally it is the worst
that can happen to a database.

It is good to keep in mind ZODB issue290 when imagining corner cases
that zconn_at has to deal with. Even though that issue is ZODB5 only, it
shows what kind of bugs it can be in zconn_at implementation for ZODB4.

Just for the reference: in Wendelin-based systems there is usually constant
stream of database ingestions coming from many places simultaneously. Plus many
activities crunching on the DB at the same time as well. And the more clients a
system handles, the more there will be level-of-concurrency increase. This
means that the problem of correctly handling concurrency issues in zconn_at is
not purely theoretical, but has direct relation to our systems.

--------

With this backport, zconn_at for ZODB4 becomes trivial and robust to implement:

https://lab.nexedi.com/kirr/wendelin.core/blob/484071b3/lib/zodb.py#L183-195

I would like to thank Joshua Wölfel whose internship helped this topic
to shape up:

https://www.erp5.com/project_section/wendelin-ia-project/forum/Joshua-internship-D8b7NNhWfz

/cc @nexedi, @jwolf083Signed-off-by: Kirill Smelkov <kirr@nexedi.com>

src/ZODB/DB.py

@@ -626,12 +626,25 @@ class DB(object):
         noop = lambda *a: None
         self.close = noop
 
+        # go over all connections and prepare them to handle last txn.abort()
+        txn_managers = set() # of conn.transaction_manager
         @self._connectionMap
-        def _(c):
-            if c.transaction_manager is not None:
-                c.transaction_manager.abort()
-            c.afterCompletion = c.newTransaction = c.close = noop
-            c._release_resources()
+        def _(conn):
+            if conn.transaction_manager is not None:
+                for c in six.itervalues(conn.connections):
+                    # Prevent connections from implicitly starting new
+                    # transactions.
+                    c.afterCompletion = c.newTransaction = noop
+                txn_managers.add(conn.transaction_manager)
+            conn.close = noop
+            conn._release_resources()
+
+        # abort transaction managers for all above connections
+        # call txn.abort only after all connections are prepared, as else - if
+        # we call txn.abort() above - some connections could be not yet
+        # prepared and with still active afterCompletion callback.
+        for transaction_manager in txn_managers:
+            transaction_manager.abort()
 
         self.storage.close()
         del self.storage

src/ZODB/__init__.py

@@ -26,3 +26,15 @@ sys.modules['ZODB.PersistentList'] = sys.modules['persistent.list']
 del mapping, list, sys
 
 from ZODB.DB import DB, connection
+
+# set of changes backported by Nexedi.
+nxd_patches = {
+    # Rework Connection MVCC implementation to always call
+    # storage.loadBefore(zconn._txn_time) to load objects.
+    # storage.load() is no longer called at all.
+    # https://github.com/zopefoundation/ZODB/issues/50
+    # https://github.com/zopefoundation/ZODB/pull/56
+    # https://github.com/zopefoundation/ZODB/pull/307
+    # ...
+    'conn:MVCC-via-loadBefore-only',
+}

src/ZODB/tests/PackableStorage.py

@@ -776,5 +776,5 @@ def IExternalGC_suite(factory):
 
     return doctest.DocFileSuite(
         'IExternalGC.test',
-        setUp=setup, tearDown=zope.testing.setupstack.tearDown,
+        setUp=setup, tearDown=ZODB.tests.util.tearDown,
         checker=ZODB.tests.util.checker)

src/ZODB/tests/dbopen.txt

@@ -61,7 +61,7 @@ While it's boring, it's important to verify that the same relationships
 hold if the default pool size is overridden.
 
     >>> handler.clear()
-    >>> st.close()
+    >>> db.close()
     >>> st = Storage()
     >>> PS = 2 # smaller pool size
     >>> db = DB(st, pool_size=PS)
@@ -117,7 +117,7 @@ We can change the pool size on the fly:
 Enough of that.
 
     >>> handler.clear()
-    >>> st.close()
+    >>> db.close()
 
 More interesting is the stack-like nature of connection reuse.  So long as
 we keep opening new connections, and keep them alive, all connections
@@ -256,7 +256,7 @@ Nothing in that last block should have logged any msgs:
 If "too many" connections are open, then closing one may kick an older
 closed one out of the available connection stack.
 
-    >>> st.close()
+    >>> db.close()
     >>> st = Storage()
     >>> db = DB(st, pool_size=3)
     >>> conns = [db.open() for dummy in range(6)]
@@ -324,7 +324,7 @@ gc to reclaim the Connection and its cache eventually works, but that can
 take "a long time" and caches can hold on to many objects, and limited
 resources (like RDB connections), for the duration.
 
-    >>> st.close()
+    >>> db.close()
     >>> st = Storage()
     >>> db = DB(st, pool_size=2)
     >>> conn0 = db.open()

src/ZODB/tests/testConnection.py

@@ -22,7 +22,8 @@ import unittest
 import transaction
 import ZODB.tests.util
 from ZODB.config import databaseFromString
-from ZODB.utils import p64
+from ZODB.utils import p64, u64, z64, newTid
+from ZODB.POSException import ConflictError
 from persistent import Persistent
 from zope.interface.verify import verifyObject
 from zope.testing import loggingsupport, renormalizing
@@ -521,17 +522,18 @@ class InvalidationTests(unittest.TestCase):
         Transaction ids are 8-byte strings, just like oids; p64() will
         create one from an int.
 
-        >>> cn.invalidate(p64(1), {p1._p_oid: 1})
-        >>> cn._txn_time
-        '\x00\x00\x00\x00\x00\x00\x00\x01'
+        >>> t = u64(cn._txn_time)
+        >>> cn.invalidate(p64(t+1), {p1._p_oid: 1})
+        >>> cn._txn_time == p64(t)
+        True
         >>> p1._p_oid in cn._invalidated
         True
         >>> p2._p_oid in cn._invalidated
         False
 
-        >>> cn.invalidate(p64(10), {p2._p_oid: 1, p64(76): 1})
-        >>> cn._txn_time
-        '\x00\x00\x00\x00\x00\x00\x00\x01'
+        >>> cn.invalidate(p64(t+10), {p2._p_oid: 1, p64(76): 1})
+        >>> cn._txn_time == p64(t)
+        True
         >>> p1._p_oid in cn._invalidated
         True
         >>> p2._p_oid in cn._invalidated
@@ -586,51 +588,29 @@ def doctest_invalidateCache():
 
         >>> connection.invalidateCache()
 
-    Now, if we try to load an object, we'll get a read conflict:
+    This won't have any effect until the next transaction:
 
-        >>> connection.root()['b'].x
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
+        >>> connection.root()['a']._p_changed
+        0
+        >>> connection.root()['b']._p_changed
+        >>> connection.root()['c']._p_changed
+        1
 
-    If we try to commit the transaction, we'll get a conflict error:
+    But if we sync():
 
-        >>> tm.commit()
-        Traceback (most recent call last):
-        ...
-        ConflictError: database conflict error
+        >>> connection.sync()
 
-    and the cache will have been cleared:
+    All of our data was invalidated:
 
-        >>> print(connection.root()['a']._p_changed)
-        None
-        >>> print(connection.root()['b']._p_changed)
-        None
-        >>> print(connection.root()['c']._p_changed)
-        None
+        >>> connection.root()['a']._p_changed
+        >>> connection.root()['b']._p_changed
+        >>> connection.root()['c']._p_changed
 
-    But we'll be able to access data again:
+    But we can load data as usual:
 
         >>> connection.root()['b'].x
         1
 
-    Aborting a transaction after a read conflict also lets us read data and go
-    on about our business:
-
-        >>> connection.invalidateCache()
-
-        >>> connection.root()['c'].x
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
-
-        >>> tm.abort()
-        >>> connection.root()['c'].x
-        1
-
-        >>> connection.root()['c'].x = 2
-        >>> tm.commit()
-
         >>> db.close()
     """
 
@@ -1244,12 +1224,16 @@ class StubStorage:
 
     def __init__(self):
         # internal
+        self._head = z64
         self._stored = []
         self._finished = []
         self._data = {}
         self._transdata = {}
         self._transstored = []
 
+    def lastTransaction(self):
+        return self._head
+
     def new_oid(self):
         oid = str(self._oid)
         self._oid += 1
@@ -1283,7 +1267,11 @@ class StubStorage:
             raise RuntimeError(
                 'StubStorage uses only one transaction at a time')
         self._finished.extend(self._transstored)
+        tid = newTid(None)
         self._data.update(self._transdata)
+        for oid,p in self._transdata.items():
+            self._data[oid] = (p, tid)
+        self._head = tid
         callback(transaction)
         del self._transaction
         self._transdata.clear()
@@ -1294,6 +1282,12 @@ class StubStorage:
             raise TypeError('StubStorage does not support versions.')
         return self._data[oid]
 
+    def loadBefore(self, oid, before):
+        if u64(before) != u64(self._head) + 1:
+            raise ValueError('noncurrent loadBefore not supported')
+        p, serial = self.load(oid)
+        return p, serial, None
+
     def store(self, oid, serial, p, version, transaction):
         if version != '':
             raise TypeError('StubStorage does not support versions.')
@@ -1302,9 +1296,12 @@ class StubStorage:
         elif self._transaction != transaction:
             raise RuntimeError(
                 'StubStorage uses only one transaction at a time')
+        serialOK = self._data.get(oid, z64)
+        if serial != serialOK:
+            raise ConflictError(oid=oid)
         self._stored.append(oid)
         self._transstored.append(oid)
-        self._transdata[oid] = (p, serial)
+        self._transdata[oid] = p
         # Explicitly returning None, as we're not pretending to be a ZEO
         # storage
         return None

src/ZODB/tests/testDB.py

@@ -89,28 +89,33 @@ def test_invalidateCache():
         >>> tm1.commit()
         >>> tm2 = transaction.TransactionManager()
         >>> c2 = db.open(transaction_manager=tm2)
-        >>> c1.root()['a']._p_deactivate()
+        >>> c2.root()['a'].value
+        1
         >>> tm3 = transaction.TransactionManager()
         >>> c3 = db.open(transaction_manager=tm3)
         >>> c3.root()['a'].value
         1
         >>> c3.close()
+
         >>> db.invalidateCache()
+        >>> c1.root.a._p_changed
+        0
+        >>> c1.sync()
+        >>> c1.root.a._p_changed
+        >>> c2.root.a._p_changed
+        0
+        >>> c2.sync()
+        >>> c2.root.a._p_changed
+        >>> c3 is db.open(transaction_manager=tm3)
+        True
+        >>> c3.root.a._p_changed
 
         >>> c1.root()['a'].value
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
-
+        1
         >>> c2.root()['a'].value
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
-
-        >>> c3 is db.open(transaction_manager=tm3)
-        True
-        >>> print(c3.root()['a']._p_changed)
-        None
+        1
+        >>> c3.root()['a'].value
+        1
 
         >>> db.close()
     """

src/ZODB/tests/test_storage.py

@@ -121,6 +121,9 @@ class MinimalMemoryStorage(BaseStorage, object):
                 end_tid = None
             else:
                 end_tid = tids[j]
+
+            self.hook(the_oid, self._cur[the_oid], '')
+
             return self._index[(the_oid, tid)], tid, end_tid
 
     def loadSerial(self, oid, serial):

src/ZODB/tests/testblob.py

@@ -54,6 +54,8 @@ except NameError:
     import io
     file_type = io.BufferedReader
 
+from . import util
+
 def new_time():
     """Create a _new_ time stamp.
 
@@ -548,10 +550,8 @@ def loadblob_tmpstore():
 
     Now we open a database with a TmpStore in front:
 
-    >>> database.close()
-
     >>> from ZODB.Connection import TmpStore
-    >>> tmpstore = TmpStore(blob_storage)
+    >>> tmpstore = TmpStore(connection)
 
     We can access the blob correctly:
 
@@ -757,7 +757,7 @@ def storage_reusable_suite(prefix, factory,
         "blob_connection.txt",
         "blob_importexport.txt",
         "blob_transaction.txt",
-        setUp=setup, tearDown=zope.testing.setupstack.tearDown,
+        setUp=setup, tearDown=util.tearDown,
         checker=zope.testing.renormalizing.RENormalizing([
             # Py3k renders bytes where Python2 used native strings...
             (re.compile(r"^b'"), "'"),
@@ -780,10 +780,10 @@ def storage_reusable_suite(prefix, factory,
     if test_packing:
         suite.addTest(doctest.DocFileSuite(
             "blob_packing.txt",
-            setUp=setup, tearDown=zope.testing.setupstack.tearDown,
+            setUp=setup, tearDown=util.tearDown,
             ))
     suite.addTest(doctest.DocTestSuite(
-        setUp=setup, tearDown=zope.testing.setupstack.tearDown,
+        setUp=setup, tearDown=util.tearDown,
         checker = ZODB.tests.util.checker + \
             zope.testing.renormalizing.RENormalizing([
               (re.compile(r'\%(sep)s\%(sep)s' % dict(sep=os.path.sep)), '/'),
@@ -823,7 +823,7 @@ def test_suite():
         "blob_tempdir.txt",
         "blobstorage_packing.txt",
         setUp=setUp,
-        tearDown=zope.testing.setupstack.tearDown,
+        tearDown=util.tearDown,
         optionflags=doctest.ELLIPSIS,
         checker=ZODB.tests.util.checker,
         ))
@@ -831,7 +831,7 @@ def test_suite():
         "blob_layout.txt",
         optionflags=doctest.ELLIPSIS|doctest.NORMALIZE_WHITESPACE,
         setUp=setUp,
-        tearDown=zope.testing.setupstack.tearDown,
+        tearDown=util.tearDown,
         checker=ZODB.tests.util.checker +
             zope.testing.renormalizing.RENormalizing([
             (re.compile(r'\%(sep)s\%(sep)s' % dict(sep=os.path.sep)), '/'),

src/ZODB/tests/testmvcc.py

@@ -33,9 +33,17 @@ This note includes doctests that explain how MVCC is implemented (and
 test that the implementation is correct).  The tests use a
 MinimalMemoryStorage that implements MVCC support, but not much else.
 
+***IMPORTANT***: The MVCC approach has changed since these tests were
+originally written. The new approach is much simpler because we no
+longer call load to get the current state of an object. We call
+loadBefore instead, having gotten a transaction time at the start of a
+transaction.  As a result, the rhythm of the tests is a little odd,
+because we no longer need to probe a complex dance that doesn't exist any more.
+
 >>> from ZODB.tests.test_storage import MinimalMemoryStorage
 >>> from ZODB import DB
->>> db = DB(MinimalMemoryStorage())
+>>> st = MinimalMemoryStorage()
+>>> db = DB(st)
 
 We will use two different connections with different transaction managers
 to make sure that the connections act independently, even though they'll
@@ -59,6 +67,10 @@ Now open a second connection.
 
 >>> tm2 = transaction.TransactionManager()
 >>> cn2 = db.open(transaction_manager=tm2)
+>>> from ZODB.utils import p64, u64
+>>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
+>>> txn_time2  = cn2._txn_time
 
 Connection high-water mark
 --------------------------
@@ -67,22 +79,20 @@ The ZODB Connection tracks a transaction high-water mark, which
 bounds the latest transaction id that can be read by the current
 transaction and still present a consistent view of the database.
 Transactions with ids up to but not including the high-water mark
-are OK to read.  When a transaction commits, the database sends
-invalidations to all the other connections; the invalidation contains
-the transaction id and the oids of modified objects.  The Connection
-stores the high-water mark in _txn_time, which is set to None until
-an invalidation arrives.
+are OK to read.  At the beginning of a transaction, a connection
+sets the high-water mark to just over the last transaction time the
+storage has seen.
 
 >>> cn = db.open()
 
->>> print(cn._txn_time)
-None
->>> cn.invalidate(100, dict.fromkeys([1, 2]))
->>> cn._txn_time
-100
->>> cn.invalidate(200, dict.fromkeys([1, 2]))
->>> cn._txn_time
-100
+>>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
+>>> cn.db().invalidate(p64(100), dict.fromkeys([1, 2]))
+>>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
+>>> cn.db().invalidate(p64(200), dict.fromkeys([1, 2]))
+>>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
 
 A connection's high-water mark is set to the transaction id taken from
 the first invalidation processed by the connection.  Transaction ids are
@@ -95,8 +105,8 @@ but that doesn't work unless an object is modified.  sync() will abort
 a transaction and process invalidations.
 
 >>> cn.sync()
->>> print(cn._txn_time)  # the high-water mark got reset to None
-None
+>>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
 
 Basic functionality
 -------------------
@@ -109,9 +119,9 @@ will modify "a."  The other transaction will then modify "b" and commit.
 >>> tm1.get().commit()
 >>> txn = db.lastTransaction()
 
-The second connection has its high-water mark set now.
+The second connection already has its high-water mark set.
 
->>> cn2._txn_time == txn
+>>> cn2._txn_time == txn_time2
 True
 
 It is safe to read "b," because it was not modified by the concurrent
@@ -153,22 +163,23 @@ ConflictError: database conflict error (oid 0x01, class ZODB.tests.MinPO.MinPO)
 This example will demonstrate that we can commit a transaction if we only
 modify current revisions.
 
->>> print(cn2._txn_time)
-None
+>>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
+>>> txn_time2  = cn2._txn_time
 
 >>> r1 = cn1.root()
 >>> r1["a"].value = 3
 >>> tm1.get().commit()
 >>> txn = db.lastTransaction()
->>> cn2._txn_time == txn
+>>> cn2._txn_time == txn_time2
 True
 
 >>> r2["b"].value = r2["a"].value + 1
 >>> r2["b"].value
 3
 >>> tm2.get().commit()
->>> print(cn2._txn_time)
-None
+>>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+True
 
 Object cache
 ------------
@@ -302,22 +313,18 @@ same things now.
 >>> r2["a"].value, r2["b"].value
 (42, 43)
 
+>>> db.close()
 
 Late invalidation
 -----------------
 
-The combination of ZEO and MVCC adds more complexity.  Since
-invalidations are delivered asynchronously by ZEO, it is possible for
-an invalidation to arrive just after a request to load the invalidated
-object is sent.  The connection can't use the just-loaded data,
-because the invalidation arrived first.  The complexity for MVCC is
-that it must check for invalidated objects after it has loaded them,
-just in case.
+The combination of ZEO and MVCC used to add more complexity. That's
+why ZODB no-longer calls load. :)
 
 Rather than add all the complexity of ZEO to these tests, the
 MinimalMemoryStorage has a hook.  We'll write a subclass that will
-deliver an invalidation when it loads an object.  The hook allows us
-to test the Connection code.
+deliver an invalidation when it loads (or loadBefore's) an object.
+The hook allows us to test the Connection code.
 
 >>> class TestStorage(MinimalMemoryStorage):
 ...    def __init__(self):
@@ -351,6 +358,12 @@ non-current revision to load.
 >>> oid = r1["b"]._p_oid
 >>> ts.hooked[oid] = 1
 
+This test is kinda screwy because it depends on an old approach that
+has changed.  We'll hack the _txn_time to get the original expected
+result, even though what's going on now is much simpler.
+
+>>> cn1._txn_time = ts.lastTransaction()
+
 Once the oid is hooked, an invalidation will be delivered the next
 time it is activated.  The code below activates the object, then
 confirms that the hook worked and that the old state was retrieved.
@@ -395,14 +408,12 @@ section above, this is no older state to retrieve.
 False
 >>> r1["b"]._p_state
 -1
->>> r1["b"]._p_activate()
+
+>>> cn1._txn_time = st.lastTransaction()
+>>> r1["b"]._p_activate() # doctest: +ELLIPSIS
 Traceback (most recent call last):
  ...
-ReadConflictError: database read conflict error (oid 0x02, class ZODB.tests.MinPO.MinPO)
->>> oid in cn1._invalidated
-True
->>> ts.count
-1
+ReadConflictError: ...
 """
 import doctest
 import re

src/ZODB/tests/util.py

@@ -61,6 +61,7 @@ checker = renormalizing.RENormalizing([
     ])
 
 def setUp(test, name='test'):
+    clear_transaction_syncs()
     transaction.abort()
     d = tempfile.mkdtemp(prefix=name)
     zope.testing.setupstack.register(test, zope.testing.setupstack.rmtree, d)
@@ -71,7 +72,9 @@ def setUp(test, name='test'):
     os.chdir(d)
     zope.testing.setupstack.register(test, transaction.abort)
 
-tearDown = zope.testing.setupstack.tearDown
+def tearDown(test):
+    clear_transaction_syncs()
+    zope.testing.setupstack.tearDown(test)
 
 class TestCase(unittest.TestCase):
 
@@ -186,3 +189,18 @@ def mess_with_time(test=None, globs=None, now=1278864701.5):
         time.time = staticmethod(faux_time) # jython
     else:
         time.time = faux_time
+
+def clear_transaction_syncs():
+    """Clear data managers registered with the global transaction manager
+
+    Many tests don't clean up synchronizer's registered with the
+    global transaction managers, which can wreak havoc with following
+    tests, now that connections interact with their storages at
+    transaction boundaries.  We need to make sure that we clear any
+    registered data managers.
+
+    For now, we'll use the transaction manager's
+    underware. Eventually, an transaction managers need to grow an API
+    for this.
+    """
+    transaction.manager.clearSynchs()